This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Synaptotagmin 1 mediates the Ca+2-dependent fusion of neurotransmitter-filled vesicles with the presynaptic terminus in neurons. While there is considerable structural and biochemical information on the synaptotagmin molecule, its precise function is still unclear;however, most atomic resolution structures conclude that the two tandem C2 domains of synaptotagmin were autonomous agents attached by a small linker. Due to our unique purification and crystallization method, we have captured human synaptotagmin 1 C2A-C2B in a conformation that shows contacts between domains as they exist prior to Ca+2 priming. The crystals we have analyzed at SSRL to this point reveal that areas of C2A involved in Ca+2 binding, phospholipid association and SNARE protein binding are distorted and probably inactivated through intra-molecular interactions with C2B. These interactions form a trigger mechanism designed to respond to changing Ca+2 concentrations. These conclusions will be essential to our understanding of fundamental neuronal function.